The present invention pertains to the transducer art and, more particularly, to a suspension system for use with very high performance transducers such as accelerometers.
An example of a prior art accelerometer design with high performance potential is described in U.S. Pat. No. 3,702,073, invented by Jacobs, issued Nov. 7, 1972, and assigned to the same assignee as the present application. This design is comprised of three primary components, namely, a proof mass assembly which is supported between upper and lower stators. The proof mass includes a moveable flapper, or reed which is suspended via flexure elements to an outer annular support member. The flapper and outer annular support member are commonly provided as a unitary, fused quartz piece.
Arcuate capacitor pick off plates are formed on the upper and lower surfaces of the flapper by means of gold deposition. In addition, upper and lower force restoring, or torquer coils are also mounted to the upper and lower surfaces of the flapper. Each torquer coil is wound on a cylindrical core and is positioned on the flapper such that the longitudinal axis of the cylinder coincides with a line which extends through the center, and is normal to the top and bottom surfaces of the proof mass assembly.
A plurality of mounting pads are formed, by acid etching and subsequent deposition of a malleable metal such as gold, at spaced intervals around the upper and lower surfaces of the outer annular support member. These contact pads mate with planar surfaces provided on the upper and lower stators when the unit is assembled.
Each stator is generally cylindrical, having a bore provided through its planar surface. Contained within the bore is a permanent magnet. The bore and permanent magnet are configured such that the torquer coil of the proof mass assembly fits within the bore, with the permanent magnet being positioned within the cylindrical form of the torquer coil. Thus, each stator permanent magnet is in magnetic circuit configuration with a magnetic field as produced by a current flowing through the corresponding torquer coil.
Also provided on the planar surface of the stators are capacitive plates configured to form capacitors with the upper and lower capacitor pick off plates on the proof mass assembly. Thus, movement of the flapper with respect to the upper and lower stators in a differential capacitance change between the capacitors formed at the upper and lower surfaces of the flapper.
In operation, the accelerometer unit is affixed to the object to be monitored. Acceleration of the object results in pendulous, rotational displacement of the flaper with respect to the outer annular support member and the upper and lower stators. The resulting differential capacitance change caused by this displacement may be sensed by suitable circuitry. The circuitry then produces a current which, when applied to the torquer coils, tends to return the flapper to its neutral position. The magnitude of the current required to "restore" the flapper is directly related to the acceleration of the accelerometer.
Accelerometers of the type described in the Jacobs patent may be subject to thermal stresses due to mismatches in the coefficient of thermal expansion of connecting materials. The use of symmetrical geometry stators tends to cancel out most of the resultant undesirable thermal strains. However, manufacturing tolerances and material instabilities may create stresses which deform, to some extent, the flapper sensing element of the accelerometer. In addition, the coefficient of thermal expansion of the proof mass, including the outer annular support member (which is preferably formed of fused quartz), is typically less than the coefficient of thermal expansion of the upper and lower stators (which are preferably formed of a metal alloy). Hence, over the operating temperature of the accelerometer, thermal stresses are created at the contact points between the proof mass assembly and the stators.
The above stresses are transmitted into the outer annular support member of the proof mass. Imperfections in the outer annular support member of the flexures may convert the resultant strain into output bias errors. In addition, the thermal stresses may result in creep and discontinuous movements at the interface of the proof mass assembly and the stators. Such undesired movements modulate the strains on the proof mass and may produce significant hysteresis errors in accelerometers intended for high performance applications.
Further, the above thermal stresses may result in movement of the torquer coils with respect to the stator permanent magnets. Such movement may produce a flux density variance between the field produced by the coils and the permanent magnets, thereby altering the sensitivity of the accelerometer. This effect is repeatable and therefore is an error source only in systems which do not compensate for stable temperature effects.
Subsequent to the Jacobs patent, attempts have been made to reduce the above described thermal stresses. For example, stators have been constructed from materials having a coefficient of thermal expansion very close to that of quartz, thereby reducing thermal stress between the proof mass assembly and the stators.
Despite such improvements, it is desirable to identify yet further means to minimize sources of inaccuracy, particularly for accelerometer applications requiring very high precision.